I like the idea of final strike because I've seen it put to (sort of) good use.
During an RP event, Camby used final strike to save sith'a'niel from a bunch of drow.
...Of course it didn't help that he was in Kazan's group, and kazan auto fled before camby finished casting of the spell, and therefore camby ended up blowing up in a room with nothing but trees (and poor kazan), and then the imm running the event probably facepalmed as he tried to patch up the event...
But final strike in the hands of players right now doesn't do anything. Someone who likes their character probably will not finalstrike unless something happens that forces him to lose interest in his character. Even if you make FS a spell obtained through RP, I still think there are better spells for an 'epic' sorceror ultimate spell, than final strike.
That said, if it is made so that a sorceror really needs to RP hard to develop his final epic spell, then maybe it can be a choice between 2-3 spells. Sort of how mercenaries specialize in one type of weapon, sorcerors would pick one out of 3 'epic' spells, one of which may be final strike. Let's say for example your choices were:
1) Final strike
2)
Mordenkainen's Disjunction3)
Dimentional Lock.htm4)
Spell TurningI bet you nobody would pick finalstrike unless the sole purpose of the character was to screw someone over. Just sayin'.